Microlens arrays are widely employed in image sensor technology, such as charged coupling device (CCD) image sensors and complimentary metal-oxide-semiconductor (CMOS) image sensors. In general, CCD, CMOS, and other types of microlens arrays transform a light pattern (i.e., an image) into an electric charge pattern.
Microlens arrays are often formed by patterning a polymer layer formed over a color filter or array, a dielectric layer, and/or other substrate features. A subsequent reflow process and/or other heating method is then performed to heat the patterned polymer, thereby creating the required shape of each microlens. Each microlens is aligned over a corresponding photo-sensor formed in the underlying substrate. Consequently, light that is incident on each microlens is focused towards a corresponding photo-sensor.
A color filter may also be positioned between the microlens array and the underlying photo-sensor array, such that incident light focused by each microlens passes through the color filter prior to reaching a corresponding photo-sensor. Accordingly, specific wavelengths or wavelength ranges may be selectively blocked/permitted from reaching one or more of the photo-sensors. The color filter may be one or more substantially continuous layers that each substantially span the entire microlens array in a lateral direction (parallel to the substrate). Alternatively, the color filter may be segmented or otherwise subdivided into a plurality of filters each configured to block/permit different wavelengths. For example, such a filter array may include filters that only transmit red light, other filters that only transmit green light, and other filters that only transmit blue light.
However, when propagating in the same medium, blue light is more refractive than green light and red light, because the wavelength of blue light is about 430 nm, whereas the wavelengths for green and red lights are 550 nm and 600 nm, respectively. Consequently, once light passes through the color filter array, the resulting blue, green and red light will refract in different amounts along the propagation path leading towards the photo-sensor array. This can result in the one or more of the different wavelengths not being accurately focused on the photo-sensors, possibly decreasing the accuracy and reliability of the microlens-photo-sensor device.